Twist structures may arise in the surface of cylindrical components in particular during machine cutting, for example grinding of the workpiece surface. Twist refers to a directional structure in the circumferential direction, in particular on shafts in a shaft/shaft seal ring system. Depending on its characteristics the twist may result in leaks between shafts and shaft seal rings.
For this reason it is known to determine the twist structure using suitable measuring methods.
A distinction is made between a macro twist and a micro twist, depending on the characteristics of the twist. The macro twist may be formed as a dressing twist, for example, which is an axially periodic, single- to multi-thread structure which circumferentially extends at an angle. However, the macro twist may also be in the form of a periodic or nonperiodic null twist, in which the threads of the twist structure are situated and terminate exactly in the circumferential direction. In addition, the macro twist may be formed as a feed twist, which is a periodic, single-thread structure that circumferentially extends at an angle and has a period length equal to the feed rate. The micro twist may in particular be a setting twist in a micro-grinding structure whose primary structural direction departs from perpendicularity with respect to the workpiece axis.
Within the scope of the invention, a macro twist (also referred to below as “twist” for short) is involved which is periodically pronounced in both the axial and circumferential directions. The periodic characteristic in the circumferential direction has an integer value, and is also referred as the “thread number.” A detailed description of the twist measuring technique according to the prior art is found in Mercedes-Benz company standard MBN 31 007-7.
A method is known from DE 197 40 141 C1 for determining a twist structure in the surface roughness of a precision machined solid or hollow cylindrical workpiece, in which multiple axially oriented sampling segments having a high axial measuring point density are carried out on a circumferential strip of interest on the workpiece at a different, although precisely known, circumferential position and with an axial position which in each case is precisely known, whereby the individual local roughness profiles of the various sampling segments according to their axial and circumferential relative position on the workpiece are printed out in close succession to one another corresponding to their position, the representation of the measurement printout being exaggerated or extended in the radial and axial directions and being compressed in the circumferential direction, thus representing a topography which is distorted with respect to the individual dimensions according to differing affinities, but which descriptively illustrates the microstructure of the surface of the circumferential strip as a winding in the oblique view.
A method is known from DE 10 2006 001 799 B4 for twist measurement on workpiece surfaces, in which measured values are obtained by sampling the workpiece surface along a line which has an axial component parallel to the axis and has a circumferential component in the circumferential direction of the axis, the line having at least one first segment and at least one second segment, the segments in the axis having different angles; and based on the measured values, at least one parameter which characterizes the twist structure is determined.
A method of the type in question for determining a twist structure in the surface roughness of a workpiece is known from the citation “J. Seewig, T. Hercke: 2nd Generation Twist Measurement, XIX IMECO World Congress Fundamental and Applied Metrology, 2009, 1957-1961, ISBN 978-963-88410-01, Lisbon, Portugal.” The method known from the citation is also referred to as a second-generation twist measuring method. In this method, multiple sampling segments which extend in the axial direction of the workpiece and which are spaced apart relative to each other in the circumferential direction are carried out in a surface region of interest on the workpiece. A first “coarse” measuring grid is composed of 72 sampling segments at the circumference, with an angular increment of 5° and a measuring length of at least 2 mm. This coarse measuring grid makes possible the description of twist structures having a maximal thread number of 15. Higher thread numbers are recorded using a second measuring grid composed of 72 sampling segments having an angular increment of 0.5° and a measuring length which is likewise 2 mm.
The basis of the evaluation is a reconstruction of the dominant twist structure from the measured data set. For this purpose, one function (composed of three additively superimposed, harmonically related cosine functions having eight free parameters) is fitted to the discrete measured data according to least squares regression analysis. With regard to the details of this analytical method, reference is made to the literature citation. The twist structure is characterized by six parameters: the twist depth Dt, the thread number DG, the theoretical conveying cross section DF per revolution DFu, the period length DP, and the percentage support surface DLv. In the known method, the parameter or parameters of the twist structure is/are determined sequentially in three steps: first, a measuring grid is selected and the sampling segment measurement is carried out. The dominant twist structure and the characteristic values are then determined.